Diabatic flows on electrically-driven radial micro-turbocompressors

Miniaturized turbomachinery arises as a promising technology which can be part of novel environmentally friendly systems, not only in the power sector but also in heating and cooling sector. Particularly, micro-turbocompressors offer a wide range of applicability in fields ranging these from hydrogen fuel cells turbocharging to renewable thermal based micro-trigeneration systems. Its relevancy has notably increased owing to the development of high-speed electric motors and proposals for using other working fluids different than air. The existence of external heat transfer flows on the main compressor flow is a performance-limiting factor identified when conceiving miniaturized turbocompressors. The resultant high surface-to-flow ratio arising from miniaturization has the consequence that the flow in the micro-compressor can no longer be taken as adiabatic. Apart from degrading the micro-compressor performance, the characterization of its actual internal efficiency is hindered due to the distortion generated in the measurements of the flow. The closeness between the electric motor and the compressor implies that part of the heat generated in the windings is transferred to the main flow. This effect is especially important at low mass flows. Although the heat transfer between hot and cold side in conventional turbochargers of internal combustion engines has been deeply studied with large temperature differences, these motor-to-compressor heat flows entail high enough specific heat losses to be characterized and studied. This is precisely why a numerical model of the inlet, impeller, diffuser and volute of a micro-turbocompressor have been developed using ANSYS CFX. Both adiabatic and constant-temperature walls conditions have been compared using different working fluids and inlet pressurization rates. Hence, the main objective of this work is to characterize the heat flux distribution along the compressor and its impact on the main flow field. The results show that inlet and impeller walls have very low impact on the global flow field when dealing with diabatic walls. Nevertheless, the specific heat flows on the diffuser and, particularly, on the volute have been proven to be notably relevant. These latter heat fluxes convey a strong cooling process when an averaged inlet-outlet temperature is selected for the walls, especially at low mass flows. Inlet pressurization greatly reduces the impact of external heat fluxes into the flow field due to its proportional increase in mass flow. Changing the working fluid to other with higher Reynolds number and lower thermal conductivity would increase the heat transfer phenomenon. However, this can be counteracted if the given homologous operating point result in a lower temperature rise due to lower rotational speeds. The apparent polytropic efficiency across a speedline can be strongly affected by the heat transfer phenomena when dealing with diabatic flows, especially at low mass flow rates. This means that the experimental characterization of a micro-turbocompressor efficiency requires to be corrected mainly due to the diffuser and volute heat exchange.